SIMILAR INVOLVEMENT OF SEVERAL BRAIN-AREAS IN THE ANTINOCICEPTION OF ENDOGENOUS AND EXOGENOUS OPIOIDS

The complete inhibitor of the enkephalin degrading enzymes, RE 101, th ylthio)butyldithio]-1-oxopropyl}-L-phenylalanine benzyl ester, which c rosses the blood-brain barrier, induced antinociceptive effects simila r to those of exogenous opiates. The almost complete absence of tolera nce and dependence after chronic administration of RE 101 is therefore due to limited stimulation of opioid receptors by 'protected' endogen ous enkephalins. In order to clarify the mechanisms involved in these response, we have investigated the participation of several brain stru ctures in the antinociceptive effects induced by systemic administrati on of morphine or RE 101. Rats were implanted with bilateral cannulae into the ventro-basal thalamus, central amygdala and periaqueductal gr ay matter, or with a cannula into the raphe magnus nucleus. The antino ciceptive responses induced by systemic morphine or RE 101 were measur ed by using the tail-electrical stimulation test, where three differen t thresholds were determined: motor response, vocalization and vocaliz ation post-discharge. The ability of the opioid receptor antagonist me thylnaloxonium to block these antinociceptive responses was evaluated after local injection into the different brain structures. The blockad e of morphine- and RE 101-induced antinociception was similar, and was stronger when methylnaloxonium was injected into the periaqueductal g ray matter and raphe magnus nucleus than when it was injected into the ventro-basal thalamus and amygdala. These results suggest that brain structures related to the control of pain seem to be the same for the antinociception induced by exogenous opiates and endogenous opioids.